The STEP power plant is a spherical tokamak designed to demonstrate net electric power. The design phase involves using plasma models to optimize fusion performance under various physics and engineering constraints. A modeling workflow, including core plasma modeling, MHD stability analysis, SOL and pedestal modeling, coil set and free boundary equilibrium solvers, and whole plant design, has been developed to specify design parameters and develop viable scenarios. The integrated core plasma model JETTO is used to develop flat-top operating points that meet fusion power performance criteria. Key plasma parameters such as normalized beta, Greenwald density fraction, auxiliary power, and radiated power have been scanned to define the operational space and derive candidate non-inductive flat-top points. The assumed auxiliary heating and current drive is either from electron cyclotron systems or a combination of electron cyclotron and electron Bernstein waves. Due to uncertainties in confinement for relevant parameter regimes, two candidate flat-top points have been developed for each of the two auxiliary heating and current drive scenarios, totaling four operating points. A lower confinement assumption suggests operating points in high-density, high auxiliary power regimes, whereas higher confinement allows access to a broader parameter regime in density and power while maintaining target fusion power performance. The STEP design aims to demonstrate a net steady-state electric power output of at least 100 MW, corresponding to an engineering fusion energy gain factor just above 1.0. The main geometric parameters planned for the STEP design include a major radius of 3.6 m, an aspect ratio of 1.8, and an elongation of 2.8. The toroidal magnetic field at the major radius is 3.2 T, and the expected plasma current during flat-top is approximately 20 MA. The STEP will be fitted with tritium breeding blankets on the low-field side for tritium fuel regeneration and protection of sensitive components. The total fusion power during flat-top operation aims for a range of 1.5–1.8 GW. The STEP design parameters are based on a balance of performance, cost, and engineering constraints. The central column has a diameter close to 3.0 m, and the design includes a low-capacity central solenoid for assisting start-up of scenarios. Flat-top operation is fully non-inductive, with current driven by a combination of auxiliary systems and the bootstrap current. Two different auxiliary heating and current drive scenario options are being assessed for the current machine design. The first option includes microwave heating and current drive with both electron cyclotron (EC) and electron Bernstein wave (EBW) systems. The second option includes EC systems only. These methods for auxiliary heating and current drive in STEP have been suggested after careful consideration of alternative methods. The STEP design also addresses challenges such as exhaust management, MHD instabilities, and plasma current and internal inductance. The paper presents the JETTO part of the modeling workflow used to guide the definition of the STEP operational space during flat-top. TheThe STEP power plant is a spherical tokamak designed to demonstrate net electric power. The design phase involves using plasma models to optimize fusion performance under various physics and engineering constraints. A modeling workflow, including core plasma modeling, MHD stability analysis, SOL and pedestal modeling, coil set and free boundary equilibrium solvers, and whole plant design, has been developed to specify design parameters and develop viable scenarios. The integrated core plasma model JETTO is used to develop flat-top operating points that meet fusion power performance criteria. Key plasma parameters such as normalized beta, Greenwald density fraction, auxiliary power, and radiated power have been scanned to define the operational space and derive candidate non-inductive flat-top points. The assumed auxiliary heating and current drive is either from electron cyclotron systems or a combination of electron cyclotron and electron Bernstein waves. Due to uncertainties in confinement for relevant parameter regimes, two candidate flat-top points have been developed for each of the two auxiliary heating and current drive scenarios, totaling four operating points. A lower confinement assumption suggests operating points in high-density, high auxiliary power regimes, whereas higher confinement allows access to a broader parameter regime in density and power while maintaining target fusion power performance. The STEP design aims to demonstrate a net steady-state electric power output of at least 100 MW, corresponding to an engineering fusion energy gain factor just above 1.0. The main geometric parameters planned for the STEP design include a major radius of 3.6 m, an aspect ratio of 1.8, and an elongation of 2.8. The toroidal magnetic field at the major radius is 3.2 T, and the expected plasma current during flat-top is approximately 20 MA. The STEP will be fitted with tritium breeding blankets on the low-field side for tritium fuel regeneration and protection of sensitive components. The total fusion power during flat-top operation aims for a range of 1.5–1.8 GW. The STEP design parameters are based on a balance of performance, cost, and engineering constraints. The central column has a diameter close to 3.0 m, and the design includes a low-capacity central solenoid for assisting start-up of scenarios. Flat-top operation is fully non-inductive, with current driven by a combination of auxiliary systems and the bootstrap current. Two different auxiliary heating and current drive scenario options are being assessed for the current machine design. The first option includes microwave heating and current drive with both electron cyclotron (EC) and electron Bernstein wave (EBW) systems. The second option includes EC systems only. These methods for auxiliary heating and current drive in STEP have been suggested after careful consideration of alternative methods. The STEP design also addresses challenges such as exhaust management, MHD instabilities, and plasma current and internal inductance. The paper presents the JETTO part of the modeling workflow used to guide the definition of the STEP operational space during flat-top. The